This invention relates to a method for constructing microlens ends for optical fibres, particularly for biomedical and/or surgical use, by using the laser light transmitted by the fibre itself in order to generate a microcrater in a suitable material (for example clay containing a certain percentage of iron oxide) and consequently to locally attain the required temperature for melting said end.
Fibres with microlens end were initially introduced for optical communication applications, for the purpose of improving the efficiency of coupling either to the source (for example a semiconductor laser) or to a second fibre. Subsequently, with the rapid development of laser applications in biomedicine and surgery, optical fibres were widely used for conveying radiation from the source to the target, and thus also allowing endoscopic applications. Often, for example for surgical applications, high powers have to be transmitted by coupling a single optical fibre to an argon laser (5-30 W) or to a Nd-YAG laser (50-100 W). In this type of application, microlens ends have proved advantageous compared with the usual flat ends both in focusing the light energy while preventing contact between the fibre and tissue, and thus reducing the risk of contaminating and damaging the fibre termination, and in utilising the increased divergence of the output light beam to obtain rapid reduction in the power density beyond the focal point and thus reduce the risk of damaging internal tissues.
For example, some tests have been carried out in experimental neurosurgery (Sottini M, et al. in Interdisciplinary Trends in Surgery, vol. 11, pp. 989-992, 1979, Minerva Medica).
Microlens ends are also used in laser endoscopy because the considerable divergence of the output beam allows sufficiently uniform treatment over relatively large areas and in some cases also allows vaporisation of large tumoral masses.
Whereas communication applications involve small-diameter monomode or multimode graded index fibres (for example with a 4-60 .mu.m core), in the case of medical applications step-index fibres with a quartz core and relatively large diameter (200-600 .mu.m) are used.
The methods for constructing microlens ends proposed up to the present time are generally directed towards the first type of application. Some interesting methods, for example, are based on selective etching. However, large core diamater fibers require a relatively long etching time. It makes this technique unattractive for the considered application, especially in view of fabricating the fiber end in the surgery theater in real time.
Another class of fabrication process makes use of thin-film deposition techniques of photoresists or other organic materials. Sometimes the deposition is followed by photoshaping of the film. These materials, however , are not convenient for high-power laser applications due to possible damaging of the microlens. Even with low- or medium-power lasers, problems in medical applications can arise from the use of materials which are not completely safe if contact of the fiber tip with biological tissue occurs; this has to be borne in mind in endoscopic surgery or in photoradiation therapy of cancer following hematoporphyrin sensitization.
A method has also been proposed which enables a microlens of high index glass to be deposited on the flat fibre end, this being obtained by melting a droplet of the required glass into a spiral filament. However, again the relatively low melting point seems to make the microlens subject to damage. Moreover, construction of the fibre end requires a relatively complicated device.
Finally, a third class of fabrication method is based on heating the fibre end to its melting point. The fibre tip then assumes the required microlens shape by the effect of surface tension. In particular, in the case of graded index fibres of glass or fused quartz, various methods have been used for heating and melting the end. Firstly, Kato (Kato D, in J. Appl. Phys., vol. 44, p. 294, 1973) used a hydrogen-oxygen torch, and then Paek and Weaver (Paek U.C. et al., in Appl. Opt., vol. 14, p. 294, 1975) used a CO.sub.2.sup..dwnarw. laser, and finally Benson et al. (Benson W.W. et al., in Appl. Opt., vol. 14, p. 2815, 1975) used a natural gas microtorch.
From the data published in the literature, it appears that all these methods allowed the construction of microlenses on fibres having a core diameter of between 40 and 150 .mu.m. Analogous results are probably obtained using arc discharges. This method has been tested experimentally in the case of a Corning graded index fibre with a 60 .mu.m diameter core. Tungsten needle electrodes were used at a distance of 1 mm. The power was 10 W for 4 sec.
It should be noted that all these methods require the use of heat sources external to the fibre. In some cases, said sources are also rather costly (for example CO.sub.2 lasers) and/or require the use of fairly complicated complementary devices.
In conclusion, none of the aforesaid methods seems to be entirely satisfactory for manufacturing microlenses for biomedical or surgical use. In particular, those methods using heating, which are relatively the most convenient, require the use of a source external to the fibre, as stated. Again, of these methods, the cheapest and least cumbersome techniques, like that of using a microtorch, may have their own disadvantages: heating is generally not uniform and as a consequence the microlens symmetry strongly depends on the operator's skill, especially for large-diameter fibres, (in this respect it should be noted that the experiments described in the literature are limited to fibres having a diamater of less than 150-200 .mu.m, whereas fibres for medical use have a core diameter of between 200 and 600 .mu.m). All this makes it difficult to immediately reconstruct, for example in the operating theatre, a microlens end which has become damaged during biomedical or surgical treatment. On the other hand, because of the high risk of this damage, such a requirement would seem to be particularly felt by potential users.